Unwind and ARM.exidx that can't be shaken off

[Chiichen]

Brief

• I'm trying to develop an application with Rust on an RTOS. I'm using armv7a-none-eabi as the target, but I've found that the compiled output always has a lot of ARM.exidx segments, which is related to unwind. I set panic = abort, and as expected, there should be no unwind-related segments in the compiled output.
• This also caused my compilation results to be extremely bloated. Only a static library with the simplest add function had a binary size of 900kb.

Version

nightly-x86_64-unknown-linux-gnu - rustc 1.81.0-nightly (8fcd4dd08 2024-06-18)

[bjorn3]

You probably need to mark it as discard in the linker script using something like the following in the SECTIONS block:

/DISCARD/ :
{
  *(.ARM.exidx .ARM.exidx.* .ARM.extab.*);
}

See cortex-m/cortex-m-rt/link.x.in at f3f85e683f98d0f44ddadcb725e6e05c51c978b1 · rust-embedded/cortex-m · GitHub

If you are using the cortex-m-rt crate an appropriate linker script should be generated automatically and it can be enabled by passing -C link-arg=-Tlink.x to rustc. It may not be appropriate for use with an RTOS though. If your RTOS provides a linker script for applications to use, make sure to pass it to the linker and add the DISCARD block if it isn't there already.

[Chiichen]

So It can't be done by rustc automatically by passing some arguments? Do you think it sounds great to implement a similar mechanism in Rust? I think this is an implicit behavior and should be replaced by a more explicit behavior.

[bjorn3]

The target specification for armv7a-none-eabi declares panic=abort and doesn't force unwind tables, so it seems to be LLVM which decided to emit unwind tables despite nothing forcing them to be present.

[Chiichen]

I found a discussion here : [llvm-dev] Is it possible to remove ARM.exidx section?. It seems that pass arguments -fno-exceptions -fno-unwind-tables -Os can remove ARM.exidx section, but I can't find out such a way to pass it to llvm in rust